Molding of sintered strontium/calcium indate and the use thereof

ABSTRACT

Alkaline earth metal indates of the formula (Sr,Ca)In 2  O 4  can be converted by sintering or fusing into compact moldings which, owing to their stability are suitable even at temperatures of at least 800° C. as reaction vessels for chemical reactions in the presence of bismuth (III) oxide and/or alkaline earth metal oxides.

DESCRIPTION

The present invention relates to moldings composed of sintered strontiumindate of the formula (Sr,Ca)In₂ O₄, to a process for the preparationthereof and to the use of these moldings as vessel material for chemicalreactions, during which bismuth oxide, if appropriate together withalkaline earth metal oxides, occur (sic) as a reactant at hightemperatures.

The compound strontium indate has been known for a fairly long time (Z.Naturforschung 19b ( 1964 ) 955 ) . CaIn₂ O₄ is isotypic with SrIn₂ O₄.The crystal structure of these indates is known (Z. Anorg. Allg. Chemie398 (1973) 24). The indates can be obtained from In₂ O₃ and an alkalineearth metal oxide by heating at 1200° C.

Bismuth oxide shows--particularly at relatively hightemperatures--markedly basic behavior. It melts at 824° C. and stronglyattacks melting crucibles and silicates. According to observations byArpe and Muller-Buschbaum (J. Inorg. Nucl. Chem. 39 (1977) 233), evenAl₂ O₃ is dissolved and converted to bismuth aluminates.

It was therefore the object to find a crucible material which isresistant at high temperatures, in particular at temperatures above 800°C., to molten bismuth(III) oxide. The present invention achieves thisobject.

The invention is based on the finding that moldings composed of sinteredstrontium/calcium indate, (Sr,Ca)In₂ O₄, are stable in the presence ofmolten bismuth(III) oxide even at high temperatures.

These moldings can be produced by compacting a fine powder of strontiumindate, calcium indate and/or solid solutions of the formula Sr_(x)Ca_(1-x) In₂ O₄ with 0<x<1, if appropriate with the addition of binders,to give a molding and heating the latter for several hours totemperatures from 1000° to 1500° C. Under these conditions, sinteringtakes place. The sintering time extends to at least 2 hours even at1500° C. At lower temperatures, the sintering takes longer. It shouldnot be continued unnecessarily, since otherwise the formation of largecrystals is promoted, which adversely affects the strength of themoldings.

Calcium indate melts above 1500° C. Strontium indate melts only above1600° C. Fine grinding of the strontium indate powder used for sinteringis advantageous. Molding can be carried out by known processes, forexample by extrusion (bars, tubes) or by compaction (uniaxial orisostatic). Strontium indate can also be fused and allowed to solidifyin molds, in order to obtain moldings.

The moldings obtained can assume the shape of spheres, tubes, ingots orbars, preferably that of reaction vessels such as crucibles, or the formof plates. Crucibles can also be produced by milling-out from an ingot.

A main difficulty in producing thin superconductive layers from thehitherto known oxidic materials on a carrier is the contamination of thesuperconductor by the carrier material. After the oxidic materials havebeen applied, a hermal aftertreatment is as a rule necessary in order toadjust the oxygen content in the superconductive layer. At this time,the superconductor reacts with the substrate material. This can bedetected by measuring the concentration profile of the cations of thecarrier material in the superconductor layer as a function of thedistance from the surface of the carrier. Layers contaminated by cationshave very poor superconductive properties (low T_(c), broadtransitions). (Sr,Ca)In₂ O₄ is inert to the known oxidic superconductormaterials (containing Bi₂ O₃, SrO, CaO, CuO) even at 1000° C. It istherefore suitable, in the form of ceramic sintered bodies or bodiessolidified from the melting point (sic), in particular in the form ofplates, or in the form of single crystals, as a substrate material inthe production of thin superconductive layers by processes such as, forexample, CVD (chemical vapor deposition), spray coating, sputtering orion beam vaporization, in which a thermal aftertreatment of the coatedsubstrates is necessary. For example, thin oxide layers (less than 5 μm)can be applied by ion beam vaporizing, sputtering, laser vaporizing orCVD, and thick oxide layers (thicker than 5 μm, in particular thickerthan 10 μm) can be applied by screen printing or plasma spraying.

Sintered indate (Sr,Ca)In₂ O₄ is also outstandingly suitable as acrucible material for the solid-state reaction of the oxides of bismuth,strontium, calcium and copper to form phases having HTsuperconductivity, and generally for producing ceramics having a contentof Bi, In or Tl. This is of interest inasmuch as the formation of thesuperconductive phase in the Bi-Sr-Ca-Cu-O system, which has atransition temperature of 110 K., takes place only at temperatures justbelow the melting point of the mixture. When such sintering experimentsare carried out in crucibles or on carrier plates of Al₂ O₃, the latterare strongly attacked and contaminated products of unsatisfactoryquality, i.e. a low content of 110 K. phase, are obtained. If, however,the reaction is carried out on a substrate of sintered strontium indate,calcium indate or a Sr-Ca indate, the content of the desired 110 K.phase is increased, under otherwise identical conditions. Furthermore,it becomes possible to work at higher sintering temperatures. It is notnecessary for the carrier to be composed of solid indate. Preferably, itis composed of a ceramic, for example. Al₂ O.sub. 3, to which a thinlayer of indate has been applied, for example by plasma spraying or byscreen printing.

In place of pure SrIn₂ O₄ or pure CaIn₂ O₄, the solid solutions of theformula Sr_(x) Ca_(1-x) In₂ O₄ with 0<x<1, in particular 0.91<x<1, inparticular 0.91<x<1 (sic), can also be used. Moldings composed of thesesolid solutions can be prepared by mixing the two indates, or startingfrom the mixture of the three oxides in an In₂ O₃ :alkaline earth metaloxide molar ratio of 1:1, by sintering or fusion, and can be used in thesame way as moldings composed of pure strontium indate.

The invention is explained in more detail by the examples.

EXAMPLE 1

SrIn₂ O₄ powder is prepared according to H. Schwarz and D. Bommert; Z.Naturforsch. 19b (1964) 955. It is then ground in acetone as thegrinding medium together with zirconiumdioxide balls (ten times thequantity by mass) in an attritor mill for 3 hours at 1000 rpm. Thegrinding balls are screened off and the solvent is distilled off in arotary evaporator. The remaining powder is dried at 80° C. It can easilybe compacted without additives to give plates or ingots, eitherisostatically or uniaxially. Further investigations have shown that thematerial sinters, starting at 1000° C.

An ingot (about 1×2×6 cm) isostatically compacted under 300 MPa has agreen density of 63% of the theoretically attainable density (density ofthe single crystal 6,907 g/cm³). The sintering is carried out inaccordance with the following program: heat to 1000° C. in 2 hours, heatfurther to 1500° C. in 5 hours, hold this temperature for 5 hours andthen cool to 100° C. in 5 hours. The sintered ingot has a density of6.413 g/cm^(s) (i.e. 93% of the theoretically obtainable density). Itcan then be worked, for example sawed into plates.

EXAMPLE 2

A plate of SrIn₂ O₄ (diameter 40 mm, 3 mm thick) prepared according toExample 1 is used as a firing base for a uniaxially compacted molding(diameter 10 mm, thickness 1 mm) composed of the mixture of the metaloxides having the empirical composition Bi₄ Sr₃ Ca₃ Cu₆ O₁₈₊α. Aftertreatment for 100 hours at 845° C., the sample is removed from thefurnace. The molding does not react with the substrate material. Meltoozing out of the molding does not wet the SrIn₂ O₄ plate. The sinteredbody does not stick to the surface of the plate after cooling. Indium isnot detectable in the sintered body. The conductivity measurement showsa superconductive transition at 105 K. (FIG. 1, measured on heating,0.01 Å). A sample sintered under the same conditions on a firing base ofalumina has a critical temperature of about 60 K. with only a smallcontent of a phase which becomes superconductive at 105 K. (FIG. 2,measured as FIG. 1).

Example 3

Solid solutions of the composition (Sr,Ca)₁ In₂ O₄ (Sr:Ca=2:1) were keptfor about 10 hours in a melt of a Bi/Sr/Ca/Cu superconductor.Energy-dispersive X-ray analyzers show no contamination of thesuperconductor by indium (FIG. 3). Only traces of bismuth and copper areafterwards detectable in the solid solutions themselves (FIG. 4).

EXAMPLE 4

3.1250 g of In₂ O₃, 1.7508 g of Bi₂ O₃, 0.8763 g CaO, 4.4736 g of SrCO₃and 2.4106 g of CuO are weighed out and intimately mixed in an agatemortar. The mixture is then pre-reacted for 6 hours at 800° C. in acorundum crucible. The temperature is then raised to 1000° C. andmaintained for 6 hours at 1000° C. This is followed by cooling to 300°C. within 4 hours, and the sample of the empirical composition Bi₀.5In₁.5 Sr₂ CaCu₂ O_(x) is removed. The sample is once more homogenized inan agate mortar and then compressed into tablets (diameter: 1 cm,thickness: about 2 mm). These are then sintered for 30 hours at 1000° C.on an Al₂ O₃ base and removed from the furnace after cooling to 100° C.(3 hours). A sample prepared in this way shows a critical temperature of110 K. (FIG. 5). Essentially SrIn₂ O₄ and Bi-containing superconductivephases are found in the X-ray diagram. The X-ray diagram of SrIn₂ O₄ isshown in FIG. 6.

A comparison of FIG. 5 with FIG. 2 shows that the presence of SrIn₂ O₄in the reaction batch has a favorable influence on the formation of the110 K. phase.

EXAMPLE 5

200 g of strontium indate are ground with 2500 g of ZrO₂ grinding ballsin ethanol as the grinding medium for 4 hours in an attritor mill at 500rpm. The grinding balls are removed by screening and the solvent isremoved in a rotary evaporator. The remaining powder is dried at 80° C.85 ml of water and 4.15 g of an oligomeric ammonium polyacrylate as adispersant are added to 200 g of ground strontium indate powder, and thewhole is mixed together with about 10 Al₂ O₃ mill stones of 20 g intotal for 24 hours on a roller block.

The resulting slip is cast into a frustoconical hollow gypsum mold. Thegypsum absorbs the water from the slip, with the formation of an indatelayer. After about 15-30 minutes, the still liquid part of the slip isdecanted again. A moist cake of 1-4 mm wall thickness remains. After afurther 1-2 hours, this crucible blank has shrunk to such an extent thatit can be taken out of the mold. It is dried for 5 hours in a desiccatorover calcium chloride and then sintered. The following temperatureprogram is used here:

Heat to 200° C. in 90 minutes; hold for 1 hour at 200° C.; heat furtherto 400° C. in 90 minutes; hold for 90 minutes at 400° C.; heat to 1200°C. in 10 hours; heat further to 1500° C. in one hour; hold for 2 hoursat 1500° C. and cool to 100° C. in 5 hours.

This gives a crucible of sintered SrIn₂ O₄, having a density of 95% oftheory.

EXAMPLE 6

Example 5 is repeated, but using Ca indate in place of Sr indate. Thetempering program for the dried crucible blank is as follows:

Heat to 150° C. in 5 hours, increase the temperature to 400° C. in afurther 5 hours (the dispersant and the moisture being removed). Thetemperature is then raised to 1200° C. in 5.5 hours and to 1500° C. inone hour and held at 1500° C. for two hours, and the product is cooledto 20° C. in five hours. To prepare the starting product, an equimolarmixture of CaO and In₂ O₃ was heated for 8 hours at 1200° C., and thepowder was homogenized in an agate mortar and then heated for a further12 hour at 1200° C.

I claim:
 1. A method for using a molding for performing a chemicalreaction in the presence of molten bismuth (III) oxide at temperaturesof at least 800° C.; said method comprising forming a reaction vesselfrom the molding wherein the molding is composed of sintered indate orindate solidified from a melt of the formula (Sr,Ca)In₂ O₄ ; andconducting the reaction in the vessel; wherein the bismuth (III) oxideis present in the vessel and does not react with the indate.
 2. A methodfor using a molding for performing a chemical reaction in the presenceof alkaline earth metal oxide at temperatures of at least 800° C; saidmethod comprising forming a reaction vessel from the molding wherein themolding is composed of sintered indate or indate solidified from a meltof the formula (Sr,Ca)In₂ O₄ ; and conducting the reaction in thevessel; wherein the alkaline earth metal oxide is present in the vesseland does not react with the indate.
 3. A method of using a molding inthe preparation of high-T_(c) -superconducting oxidic material fromoxidic material comprising the oxides of Bi, Sr, Ca and Cu, said methodcomprising reacting the oxidic material in the molding, wherein themolding is of compact indate of the formula (Sr,Ca)In₂ O₄, said reactingcomprising thermally treating the oxidic material in the molding at atemperature≧800° C; and no reaction occurs between the oxidic materialand the indate.
 4. A method as claimed in claim 3, wherein the moldingis prepared by compacting a fine powder selected from the groupconsisting of calcium indate, strontium indate, an indate of the formulaSr_(x) Ca_(1-x) In₂ O₄ with 0<x<1 and mixtures thereof; and, heating thecompacted fine powder for a sufficient time at a temperature from 1000to 1500° C.
 5. A method as claimed in claim 1, wherein the sinteredindate is prepared by sintering or fusing a pulverulent equimolarmixture of In₂ O₃ and an alkaline earth metal oxide selected from thegroup consisting of CaO and SrO.
 6. A method as claimed in claim 3,wherein the molding has the shape of a plate.
 7. A method as claimed inclaim 6, wherein the contacting of the oxidic material with the moldingis by coating the oxidic material as a layer onto the surface of theplate.
 8. A method as claimed in claim 4, wherein the fine powderadditionally comprises a binder.